1) Field of the Invention
The present invention relates to the joining of structural members and, more particularly, relates to friction stir welding of joints including at least one shim disposed between structural members.
2) Description of Related Art
Aerospace structural assemblies, such as wings for aircraft, typically include outer flexible sheets, or skins, that are supported in a desired contour to define the outer surface of the structure. According to one conventional method of manufacturing such structures, a number of stringers are connected to the skins to stiffen the skins and facilitate connection of the skins to an underlying support framework of the structure. The stringers, which are typically long angles, channels, or other types of beams, can be connected to the skins by riveting or welding while the skins are supported in a generally flat configuration. Thereafter, the stringers and the skins can be adjusted to the desired configuration, and the stringers are connected to spars or other members of the support framework.
In some applications, however, it is desirable to have skins that are relatively stiff. The stiffness of the skins prevents the skins from being readily flexed to the desired configuration during assembly. Therefore, the skins are formed to the desired contour. For example, the skins can be bent or stretched, or each skin can be formed of multiple portions that are connected in the desired configuration. Nevertheless, even small differences between the configuration of the formed skins and the underlying support structure can complicate the assembly or reduce the quality of the finished structure. For example, the stiffness of the skins can prevent or complicate the adjustment of the skins to accurately conform to the support framework. Small gaps or mismatches can result between the skin and the framework and/or between the different portions of the skin that are connected to the support framework. Such gaps are typically filled with putty or caulk, but the strength, durability, and/or corrosion resistance of the connections can be affected.
Therefore, there exists a need for an improved method for forming structural assemblies, especially aerospace structural assemblies that include outer contoured skins. The method should be capable of reducing the gaps between the members of the structure and improving the quality of the connections formed therebetween.